The transforming growth factor-beta (TGF-beta) superfamily contains a variety of growth factors that share common sequence elements and structural motifs. These proteins are known to exert biological effects on a large variety of cell types in both vertebrates and invertebrates. Many of members of the superfamily perform important functions during embryonic development in pattern formation and tissue specification and can influence a variety of differentiation processes, including adipogenesis, myogenesis, chondrogenesis, cardiogenesis, hematopoiesis, and epithelial cell differentiation. The family is divided into two general branches: the BMP/GDF and the TGF-beta/Activin/BMP10 branches, whose members have diverse, often complementary effects. By manipulating the activity of a member of the TGF-beta family, it is often possible to cause significant physiological changes in an organism. For example, the Piedmontese and Belgian Blue cattle breeds carry a loss-of-function mutation in the GDF-8/myostatin gene that causes a marked increase in muscle mass. Grobet et al., Nat. Genet. 1997 September; 17(1):71-4. Changes in fat, bone, cartilage, muscle and other tissues may be achieved by agonizing or antagonizing signaling that is mediated by an appropriate TGF-beta family member. Thus, there is a need for agents (e.g., polypeptides) that function as potent regulators of TGF-beta signaling.